This is a proposal to continue and conclude a randomized, double-blind clinical trial of the efficacy of calcium carbonate supplements in preventing neoplastic polyps (adenomas) of the large bowel in persons at high risk for these tumors. It is a collaborative investigation with Dartmouth as the coordinating center, and six clinical centers: Cleveland Clinic, Dartmouth-Hitchcock Medical Center, University of California at Los Angeles, University of Iowa, University of Minnesota, and University of North Carolina. The study involves over 850 patients enrolled shortly after diagnosis of a large bowel adenoma, with the removal of all known polyps from the bowel. They have been assigned at random to receive either calcium carbonate (providing 1200 mg per day of elemental calcium) or an identical placebo. Participants come to the clinical centers or are contacted by telephone or mail at six-month intervals for capsule distribution and assessment of compliance and toxicity. At 12 months and 48 months after entry participants receive a complete colonoscopy with removal and microscopic examination of all polyps. The primary statistical analysis will assess treatment effects by comparing the proportions of patients who develop a new adenoma in the three years between the 12-month and 48-month follow-up examinations. Secondary aims are to investigate the relationship between calcium treatment assignment and the size and number of the recurrent adenomas, to investigate the association between dietary fat and calcium intake and adenoma recurrence, and to assess the impact of baseline dietary fat and calcium intake on the efficacy of the supplement. A secondary aim added in this proposal is to assess the relationship between serum vitamin D and adenoma recurrence. The study has exceeded its original enrollment goal and the cooperation of subjects in taking pills and attending follow-up examinations has been excellent. This request is for the final four years of the planned nine-year study. Three years of additional follow-up are required, and one year for final activities and analysis.